Compositor - File Output - PNG compression ratio issue
The difference comes from the dithering. This post-process is not applied on the viewer node but is applied on the render. You can find more info on dithering : https://docs.blender.org/manual/en/dev/render/output/settings.html#dithering


  Dithering is a technique for blurring pixels to prevent banding that
  is seen in areas of gradients, where stair-stepping appears between
  colors


Dithering adds noise. It explains why file sizes are not the same (noise is harder to compress).
Below, the first image does not have dithering and the second one does have dithering (maybe you need to zoom in to see the difference).

 

To get back to my initial question, the image saved with the File Output node via the Compositor has dithering and the image saved from the Viewer Node via the Image Editor does not have dithering.

I get the info from developers on ticket I opened :
https://developer.blender.org/T74071